1. FIELD OF THE INVENTION
The present invention relates to a pulverized coal burner and a boiler including such a burner, and to a method of burning pulverized coal, and more particularly is concerned with reducing the nitrogen oxides (which will be abbreviated to "NO.sub.x ") produced at the time of the combustion of coal.
2. DESCRIPTION OF THE PRIOR ART
The NO.sub.x produced at the time of the combustion of pulverized coal almost entirely comes from the oxidization of nitrogen contained in the coal. The nitrogen in the coal is decomposed and released at the pyrolysis in the initial combustion into hydrogen cyanide (HCN) or ammonia (NH.sub.3 ). These are oxidized into NO.sub.x, but also can have the effect of reducing NO.sub.x to N.sub.2 under a condition of low oxygen concentration.
A typical combustion method is a two-stage one in which an excess fuel (air lean) combustion is performed in a burner unit so that the residual combustible components at the downstream of the flame are burnt out with air for complete combustion. This method is effective for reducing NO.sub.x and is adopted in many boilers but requires a large combustion chamber for mixing and burning all the combustion air and the combustible components, so that the boiler is large in size. As a result, the combustion has to be effected in the burner using an amount of air substantially necessary for complete combustion, thereby to improve the combustion efficiency in the flame formed in the burner and reduce NO.sub.x. Such burners are disclosed in JP-A-60-226609 and JP-A-62-276310; in which the combustion air is divided into primary, secondary and tertiary air flows and in which the mixing between the tertiary air for the complete combustion and the excess fuel flame a the central portion is delayed to facilitate the formation of a reducing atmosphere for NO.sub.x in the central portion of the flame. On the other hand, a burner for controlling the mixing state of the fuel and the air by sliding a fuel supply tube is disclosed in JP-A-55-17060, JP-A-56-44505 and JP-A-56-119406.
In order to realize the excess fuel combustion methods described above, the pulverized coal has to be ignited with as little air as possible before the tertiary air is mixed. Moreover, the fuel to be supplied from the coal nozzle has to be divided in the combustion chamber into (a) a fuel to be supplied to the central portion of the flame so as to form the reducing atmosphere for NO.sub.x, and (b) a fuel to be supplied as a heat source for promoting the pyrolysis of the coal supplied to the NO.sub.x reducing atmosphere at the central portion of the flame to the outer circumference of the flame to mix the combustion air and the coal thereby to activate the combustion. For this, it is necessary to control the coal concentration at the exit of the coal duct.
A method of changing the coal distribution in the pulverized coal duct, with the aforementioned point in mind can be classified into the following two types: (i) the carrier air flowing through the coal duct is swirled to change the distribution of the coal, as disclosed in JP-A-57-12209, JP-A-63-87508 and JP-A-54-159741; and (ii) the distribution of the coal is changed by using a venturi for narrowing the central portion of the passage of the coal duct and then diverging the downstream passage, as disclosed in JP-A-54-159742, JP-A-60-202204 and JP-A-62-172105.
In the method (i), the coal is concentrated in the vicinity of the inner circumference of the coal duct by the swirling flow of the carrier air, and a concentric passage is arranged downstream to reduce the coal concentration- at the central portion of the coal duct and to enrich the coal concentration of the coal duct of the outer circumferential passage. This method can control the coal concentration but changes the flow rate distribution of the coal carrying air. Since the carrier air is also swirled and concentrated at the inner circumference of the coal duct, the injection velocity at the central portion of the coal nozzle is decelerated but that at the outer circumference is accelerated. According to the coal injection of this method, the coal injected at a low velocity into the central portion is influenced by the swirls, which are generally added to the combustion air injected from the outer circumference of the coal nozzle, so that the coal is spread radially outwardly from the burner axis. Since such behaviour of the coal acts to reduce the coal supplied to the aforementioned NO.sub.x reducing atmosphere and accordingly to block the reactions of the NO.sub.x reducing atmosphere, there arises a problem from the view point of the low NO.sub.x combustion of the coal.
In the method (ii), the coal and the carrier air are accumulated at the central portion of the venturi so that the distribution of the coal may be controlled by making use of the inertia of the coal at the downstream diverging portion. According to this method, the coal to be injected from the central portion of the coal nozzle and supplied to the NO.sub.x reducing atmosphere is increased and injected from the outer circumference of the coal nozzle so that the coal to be supplied to the outer circumference of the flame is reduced. As a result, the fuel in the outer circumference of the flame is reduced, becoming poorly ignitable. On the other hand, the fuel to be supplied to the NO.sub.x reducing region is delayed in its mixing with the combustion air. If the fuel in this reducing region is increased, the ratio of the unburned fuel at the exit of the combustion chamber is increased, lowering the boiler efficiency.
In the light of the present invention, described below, certain features of the prior art disclosures, including some mentioned above, are now mentioned more particularly:
In JP-A-54-159741, the flow is swirled by vanes in the coal duct slightly upstream of its exit. The radially outer parts of the flow then pass through a converging annular exit passage which accelerates them.
JP-A-56-44505 shows swirl vanes at the exit of the coal duct, which then has a divergent exit flare to give highly divergent coal distribution.
In JP-A-57-12209, an upstream cyclone throws coal concentration towards the outside of the coal duct. The radially outer flow is separated from the inner by a concentric conical tubular insert at the convergently conical exit, and accelerated by wedge-shaped spacers between the insert and the outer tube which gradually reduce the available flow cross-section for the outer flow portion towards the exit.
JP-A-62-172105 shows a coal duct with an outwardly divergent transition to a larger diameter section near the exit, and flaring further at the exit. The initial outward divergence gives a relative enrichment of coal in the centre of the exit flow.
In JP-A-60-86311, the exit of the primary passage has a radially-inwardly projecting toothed flange, which is described as inhibiting radial divergence of coal after leaving the exit. JP-A-64-57004 shows the same feature.
JP-A-63-87508 also shows a radially inwardly projecting flange at the coal duct exit, following divergent transition to a larger diameter end portion.
JP-A-1-256708 shows a similar exit flange after a divergence. Because the divergence gives a reduced coal content at the outer part of the flow, the flange is concerned primarily with affecting the flow of air.
The NO.sub.x reducing performance according to the prior art as described is thought insufficient for the environmental regulations which become severer and severer day by day.